1. Field of the Invention
The present invention relates to the structure of a photodiode used in an optical transceiver module for an optical communication system in which one optical fiber is used for carrying out transmission and reception of signals by using two different wavelengths, xcex1 and xcex2 (xcex1 less than xcex2). The photodiode can detect light having a shorter wavelength, xcex1, by dexterously eliminating the influence of outgoing light having a longer wavelength, xcex2. It should be noted that the photodiode is for detecting light having a shorter wavelength, not for detecting light having a longer wavelength. To be blocked is the light having a longer wavelength, not the light having a shorter wavelength.
2. Description of Related Arts
When one optical fiber is used for both transmission and reception of signals, a laser diode (LD), which emits outgoing light, and a photodiode (PD), which receives incoming light, are placed usually in the same housing or on the same platform. Similarly, when one optical fiber is used for transmitting signals unidirectionally by using two or more different wavelengths, two or more PDs are placed usually on the same platform. A PD is a device having high sensitivity. An LD emits intense outgoing light to transfer signals to a distant place. Although the PD and LD act at different wavelengths, the PD has sensitivity to the outgoing light and detects it. When an LD and a PD are placed on the same terminal, if the PD detects the outgoing light emitted by the LD, this phenomenon is called optical crosstalk. The outgoing light acts as noise for the PD. When the PD detects the outgoing light, incoming light cannot be detected accurately. Therefore, it is necessary to minimize the crosstalk between the PD and LD. Undoubtedly, there can be electrical crosstalk between a transmitter and a receiver caused by the magnetic coupling between their electric circuits. However, to be solved here is the problem of optical crosstalk.
Researchers and engineers have devised various types of transceivers that carry out transmission and reception of signals over one optical fiber. Those transceivers employ different methods for separating the outgoing light and incoming light. The most popular method uses an optical wavelength demultiplexer to branch the path for the outgoing light and the path for the incoming light. Such a method in which the two paths are separated spatially can solve the problem of optical crosstalk relatively easily. There is a rather special transceiver module in which a PD and an LD are arranged in a straight line. Such a method in which almost the same path is used for both transmission and reception makes it difficult to solve the problem of optical crosstalk.
FIG. 1 shows a system of simultaneous bidirectional optical communication in which one optical fiber connects a central office and a subscriber for carrying out bidirectional signal transmission by using two different wavelengths, xcex1 and xcex2. This is a system of wavelength division multiplexing (WDM) bidirectional communication. A central office generates a signal using LD1 and sends it to PD2 at a subscriber via an optical fiber 1, an optical wavelength demultiplexer 2, an optical fiber 3, an optical wavelength demultiplexer 4, and an optical fiber 5. The subscriber generates another signal using LD2 and sends it to PD1 at the central office via an optical fiber 6, the optical wavelength demultiplexer 4, the optical fiber 3, the optical wavelength demultiplexer 1032, and an optical fiber 7. Thus, signals can be transmitted in opposite directions at the same time over one optical fiber.
At the central office, the optical wavelength demultiplexer 2 is connected to the optical fibers 7 and 1 to separate an upstream signal and a downstream signal according to their wavelengths. The outgoing light carrying downstream signals has the wavelength xcex2 and the incoming light carrying upstream signals has the wavelength xcex1. The downstream and upstream signals travel over the one optical fiber 3 at the same time.
At the subscriber, the optical wavelength demultiplexer 4 is connected to the optical fibers 5 and 6 to separate incoming light and outgoing light. The photodiode PD2 receives incoming light having the wavelength xcex2, and LD2 generates outgoing light having the wavelength xcex1. Although not shown, there are individual electric circuits beyond PD2 and LD2.
The present invention addresses problems related to a transceiver at a central office. Problems at a central office are different from those at a subscriber because the aspect related to wavelength is reversed. In the example shown in FIG. 1, the optical wavelength demultiplexers 2 and 4 spatially separate the optical paths of the outgoing light and the incoming light. Therefore, the problem of optical crosstalk can be solved by the improvement of the performance of the optical wavelength demultiplexer, for example. The problem to be solved by the present invention is the optical crosstalk between the PD and LD at a central office.
FIG. 2 shows another system in which two signals are transmitted unidirectionally from a central office to a subscriber by using different wavelengths, xcex1 and xcex2. This is a system of WDM unidirectional communication. A central office generates two signals by using two different LDs. They are combined at an optical wavelength multiplexer 8 and transmitted from the central office to a subscriber over one optical fiber. At the subscriber, the two signals are separated according to their wavelengths by an optical wavelength demultiplexer 4. The photodiodes PD1 and PD2 selectively receive the signals. Here, crosstalk between PD1 and PD2 also poses a problem.
FIG. 3 shows a typical example of a conventional PD module used as a receiver in an optical communication system in which the optical paths are separated as shown in FIGS. 1 and 2. This type of PD module is still mainly used. Lead pins 9 are provided at a circular metal stem 10, at the center of which a submount 11 supports a PD chip 12. A lens 13 is attached to a cylindrical cap 14 welded to the stem 10 in alignment. A cylindrical sleeve 15 is placed on the cap. A ferrule 16 is inserted into the mandrel hole of the sleeve 15. The ferrule 16 supports the end of an optical fiber 17. The tip of the ferrule 16 is polished on the skew. The sleeve 15 is covered with a bend limiter 18 to protect the optical fiber 17. This explains the structure of a PD module currently in use. The systems shown in FIGS. 1 and 2 also include LD modules. In an LD module, only the PD shown in FIG. 3 is replaced by an LD. Therefore, an LD module has a structure similar to that of a PD module, and no explanation about it is provided here. PD modules and LD modules now in use employ a metal case, so that optical fibers are arranged stereoscopically (three-dimensionally). Although high in performance, those modules require centering work at the time of assembly. The centering is a time-consuming job and increases the manufacturing cost. Because of the high price, those modules are not suitable in achieving widespread application.
Researchers and engineers have been energetically studying surface-mounted types for use as less costly PD modules, LD modules, or PD-LD modules. FIG. 4 shows an example of a conventional surface-mounted-type module that uses a back-illuminated-type PD. A rectangular silicon platform 19 is provided with a longitudinal, V-shaped groove 20 at the center. The groove is formed by etching. A slanted mirror face 21 is provided at the end of the V-shaped groove 20. The etching work simultaneously forms the mirror face 21 also. A PD chip 23 is fixed directly above the end portion of the V-shaped groove 20. The PD chip 23, a back-illuminated-type PD, is provided with a photo-sensitive area 24 at the upper zone. The light emerging from an optical fiber 22 propagates in the V-shaped groove 20 in parallel with the surface of the silicon platform, is reflected upward by the mirror face 21, enters the PD 23 at the back side, and reaches the photo-sensitive area 24. A surface-mounted-type module has no centering portion. Elimination of centering work accomplishes easy manufacturing.
Both the PD modules shown in FIGS. 3 and 4 can be used for detecting the incoming light separated by the optical wavelength demultiplexers shown in FIGS. 1 and 2. An optical wavelength demultiplexer can be produced, for example, by forming a wavelength-selective branching waveguide on a silicon platform. There is also a prism-type optical wavelength demultiplexer as shown in FIG. 5. A dielectric multilayer film 27 is deposited on the oblique face of transparent triangular-column glass blocks 25 and 26 for wavelength selectivity. For example, when light emerges from an optical fiber 28, the light having a specific wavelength is reflected and the other light having a different wavelength is transmitted. In FIG. 5, however, the wavelength selectivity is used for distinguishing the outgoing light from the incoming light. More specifically, the incoming light (wavelength: xcex2) emerging from the optical fiber 28 is reflected by the multilayer film 27 and introduced into a PD 30. The outgoing light (wavelength: xcex1) emitted from an LD 29 passes through the multilayer film 27 and enters the optical fiber 28.
However, the present invention whose intention is to minimize the optical crosstalk can be most suitably applied to a transceiver module in which the optical paths are not separated by an optical wavelength demultiplexer. Such a module is called an optical path non-separated type in the present invention in order to distinguish from the foregoing optical path-separated type. In the optical path non-separated type, a PD is placed at the side of the optical fiber and an LD is placed in line with the optical fiber. This type requires no optical wavelength demultiplexer. This is advantageous because the size becomes smaller and the structure becomes simpler. On the other hand, this type has a common optical axis for outgoing light and incoming light. As a result, the problem of optical crosstalk becomes more serious.
FIG. 6 shows an example of an optical path non-separated type for a module at a subscriber. The wavelengths of the outgoing light and incoming light are opposite to those at a central office. Although not shown, there is a silicon platform in a housing 31. An optical fiber 32 is housed longitudinally. An LD 33 is mounted opposite to the end of the optical fiber 32. A WDM filter 35 is provided at some point in the optical fiber 32 near its end to carry out wavelength distinction. A PD 34 is placed directly above the WDM filter 35. The outgoing light (wavelength: xcex1) emitted by the LD 33 is as powerful as 1 mW, for example. The outgoing light propagates to the outside through the optical fiber 32. The incoming light (wavelength: xcex2) having propagated through the optical fiber 32 from the outside is reflected by the WDM filter 35, enters the PD 34 at the back side, and is detected by a photo-sensitive area 36. Whereas the outgoing light is intense, the incoming light is weak. The outgoing light propagates to the WDM filter 35 through the same optical fiber 32 in the direction opposite to the incoming light. When passing through the WDM filter, part of the outgoing light may enter the PD. This intruding light causes the optical crosstalk. Notwithstanding the small percentage, the intruding light becomes an unignorable noise in comparison with the intensity of the incoming light, because the outgoing light is intense and the incoming light is weak.
FIG. 7 shows a conventional PD that has a wide range of sensitivity. When this type of PD is used, the problem of optical crosstalk becomes more serious. The structure of the InP-based PD shown in FIG. 7 is based on an epitaxial wafer in which an n-InP buffer layer 38, an n-InGaAs absorption layer 39, and an n-InP cap layer 40 are laminated on an n-InP substrate 37. At the upper zone of the PD, a p-type region 41 and a p electrode passivation layer 44 are formed. On the bottom surface, a ring-shaped n electrode 45 and an anti-reflection layer 46 are provided. When such a PD is used as a photodetector, the level of the noise caused by the outgoing light becomes higher than the signal level of the incoming light. In other words, the signal/noise ratio (S/N ratio) becomes smaller than one. When an ordinary PD, which has sensitivity to both xcex2 and xcex1, is used, the foregoing undesirable phenomenon occurs.
FIG. 8 is a graph showing the sensitivity characteristics of the PD shown in FIG. 7. The P portion in the shorter wavelength region, in which the sensitivity decreases with decreasing wavelength, corresponds to the bandgap of the InP substrate. The light having a shorter wavelength than that corresponding to the bandgap is not detected because it is absorbed by the InP substrate. The R portion in the longer wavelength region, in which the sensitivity decreases with increasing wavelength, corresponds to the bandgap of the InGaAs absorption layer. The light having a longer wavelength than that corresponding to the bandgap is not detected because its energy is lower than the bandgap of the absorption layer. In other words, the PD has sensitivity in a wide range of Q from the bandgap wavelength P of the InP substrate to the bandgap wavelength R of the InGaAs absorption layer. Therefore, the PD has sufficient sensitivity not only at the 1.3-xcexcm band but also at the 1.55-xcexcm band.
As described above, the PD having a conventional structure as shown in FIG. 7 has sensitivity in a wide range of 1.0 to 1.65 xcexcm as shown in FIG. 8. It is advantageous to have sensitivity in a wide range as above because the same PD can be used for both the 1.3-xcexcm band and 1.55-xcexcm band. Therefore, the PD having a structure as shown in FIG. 7 is most widely used for the long-wavelength light employed in optical communications. However, when the PD is used for a transceiver module, the PD also detects the outgoing light in addition to the incoming light, which means that the outgoing light acts as noise. Consequently, the PD is disadvantageous in that optical crosstalk occurs between the outgoing light and incoming light.
In the transceiver module shown in FIG. 6, not all the intense outgoing light (wavelength: xcex1) emitted from the LD placed on the silicon platform (silicon bench) enters the optical fiber. The light emitted from the LD spreads out at a considerably wide angle. Some of the light strikes the silicon platform and plastics to be scattered. The silicon platform is transparent to the outgoing light. The outgoing light having entered the space made by the silicon platform and transparent plastics passes through the silicon, is reflected, and is scattered. Various complicated scattered rays of light are produced according to the distribution of the plastics, the shape of the silicon platform, and the arrangement of the other devices. When looked from the PD, the entire silicon platform shines brightly due to the scattering of the outgoing light. Such components of the outgoing light that enter the PD through various paths other than the designed path are called xe2x80x9cscattered lightxe2x80x9d or xe2x80x9cstray light.xe2x80x9d
Some components of the outgoing light enter the PD from various directions and at various heights. They enter the PD at the back side, at the front side, and at the side face. Such components of the outgoing light that enter the PD without entering the optical fiber cause the crosstalk. Such crosstalk caused by the scattered light (stray light) that does not pass through the WDM filter cannot be suppressed by the improvement of the performance of the WDM filter. When the output power of the LD is increased, the outgoing light propagating through the optical fiber increases the amount of the leakage at the WDM filter. The component of the outgoing light emitted from the LD that enters the PD after being refracted and reflected at the WDM filter is called xe2x80x9cleakage light.xe2x80x9d
The unexamined Japanese patent publication (Tokukaihei) No.4-213876 entitled xe2x80x9cPhotodetectorxe2x80x9d proposes a photodetector that is a PD comprising two stages of absorption layers. A layer structure that absorbs 1.55-xcexcm light is provided on an InP substrate and a p electrode is provided on the layer structure. On part of the layer structure, another layer structure that absorbs 1.3-xcexcm light is provided and another p electrode is provided on this layer structure. A common n electrode is provided on the bottom surface of the InP substrate. Consequently, the photodetector has a two-stage structure in which PD1 for absorbing the light having xcex1 (1.3 xcexcm) is placed at the top and PD2 for absorbing the light having xcex2 (1.55 xcexcm) is placed at the bottom.
The light having xcex2 and the light having xcex1 enter the photodetector at the front side. Since the light having xcex2 has a longer wavelength, it passes through the upper layer structure and reaches the lower layer structure to generate optical current there. In other words, PD2 absorbs the light having xcex2 at the bottom. The light having xcex1, which is shorter, is absorbed by PD1 in the upper structure to generate optical current there. In other words, PD1 can detect the light having xcex1 at the top. In order to prevent the penetration of the light having xcex1 into the lower structure, a layer having a thickness of d=mxcex1/(2n), where m is a plus integer and n is a refractive index, is provided between PD1 and PD2. The object of this layer is to reflect the light having xcex1 upward so that the light having xcex1 cannot enter PD2. Hence, this layer is called a xe2x80x9cselective reflection layer.xe2x80x9d If the light having xcex1 enters PD2, the light causes PD2 to generate optical current, so that crosstalk occurs. The selective reflection layer is provided to prevent this type of crosstalk.
However, this patent application provides no preventive measure against crosstalk in the opposite case. Such a case is out of its expectations. There is no measure against the phenomenon that the light having xcex2 is reflected by the n electrode at the bottom, returns to PD1, and adversely affects its performance. Since the selective reflection layer provided between PD1 and PD2 reflects the light having xcex1 but transmits the light having xcex2, the light having xcex2 reflected at the bottom face can pass through the layer upward.
Another unexamined Japanese patent publication, (Tokukaihei) No.9-166717, entitled xe2x80x9cOptical receiver module and optical transceiver modulexe2x80x9d proposes a photodetector for a system in which two signals having different wavelengths, xcex1=1.3 xcexcm and xcex2=1.55 xcexcm, are transmitted through one optical fiber. A first photodiode, PD1, absorbs the light having xcex1 and transmits the light having xcex2. A second photodiode, PD2, placed behind PD1, absorbs the light having xcex2. Two independent PDs are combined in tandem. They are not such composite devices as described above. The photodiode PD1 has an absorption layer that has an intermediate bandgap wavelength as expressed in xcex1 less than xcexg less than xcex2, where xcexg represents the bandgap wavelength of the absorption layer. Since xcexg is longer than xcex1, the absorption layer absorbs and detects the light having xcex1. The light having xcex2 passes through PD1 and is detected by PD2. FIG. 9 shows the structure of the PD for absorbing the light having xcex1 proposed by Tokukaihei No.9-166717. The PD is placed in an intermediate place to transmit the light having the longer wavelength. For this purpose, the PD has another opening at the side opposite to the light-entering face to allow the light having the longer wavelength to leave the PD. The PD can be called a dual opening type, because it has openings at both sides for transmitting light.
An n-InP buffer layer 51, an n-InGaAsP absorption layer 52 (xcexg=1.42 xcexcm), and an n-InGaAsP window layer 53 are grown epitaxially on an n-InP substrate 50. At the center portion, Zn is diffused to provide a p-type region 54. The center portion of the p-type region 54 is covered by an anti-reflection layer 56. Around the anti-reflection layer 56, a ring-shaped p electrode 55 is provided. At the outside of the p electrode 55, a passivation layer 57 is formed to protect the edge portion of the pn junction. A ring-shaped n electrode 58 is formed on the bottom surface of the InP substrate 50. The inside of the n electrode 58 forms an opening and is covered by an anti-reflection layer 59. Both the front and back sides have openings for transmitting light. The ring-shaped electrodes are provided without overlapping with these openings. The anti-reflection layers are provided at the openings to prevent incident light from attenuating due to reflection.
FIG. 10 shows a transmittance spectrum of the InGaAsP absorption layer 52 (xcexg=1.42 xcexcm). The mixing ratio of its quaternary mixed crystal is decided for the bandgap wavelength to take an intermediate value between 1.3 xcexcm and 1.55 xcexcm. The measured result proves the design concept. The light having a wavelength shorter than 1.4 xcexcm is absorbed almost completely, which means that the light practically does not pass through the layer. A wavelength of 1.42 xcexcm forms the boundary condition. Almost one hundred percent of the light having a wavelength longer than 1.5 xcexcm passes through the absorption layer. The transmittance varies with the thickness. The absorption layer has an enough thickness so that the light having a shorter wavelength can be absorbed completely.
The present invention intends to prevent a PD that detects the light having a shorter wavelength from suffering the crosstalk caused by the light having a longer wavelength. In the explanation below, the shorter wavelength, xcex1, is supposed to be 1.3 xcexcm and the longer wavelength, xcex2, to be 1.55 xcexcm in order to specifically show the relation between the two wavelengths. In the present invention, however, xcex1 is in the range of 1.2 to 1.38 xcexcm, and xcex2 is in the range of 1.45 to 1.65 xcexcm. At a central office, if a PD as shown in FIG. 7, which usually has sensitivity in a wide range, is used as a photodetector, it also detects the scattered light and leakage light of the outgoing light. At a central office, the incoming light has a wavelength of 1.3 xcexcm, and the outgoing light, 1.55 xcexcm. This combination of wavelengths is advantageous in eliminating the effect of the outgoing light. Dexterous exploitation of the basic properties of the semiconductor enables the production of a PD for a central office that detects the incoming light (xcex1=1.3 xcexcm) but does not detect the outgoing light (xcex2=1.55 xcexcm). This can be accomplished by selecting the bandgap wavelength xcexg of the absorption layer of the PD to satisfy the following formula:
xcex1 (incoming light) less than xcexg less than xcex2 (outgoing light).
This is possible because the two wavelengths at a central office have such an advantageous relationship. If the bandgap wavelength xcexg of the absorption layer is decided to be 1.35 to 1.45 xcexcm, for example, then the absorption layer should have a desirable quality that it detects the incoming light but does not detect the outgoing light. A bandgap wavelength can be adjusted to 1.35 to 1.45 xcexcm by using a quaternary mixed crystal of InGaAsP. In the present invention, however, xcexg is in the range of 1.3 to 1.5 xcexcm.
FIG. 10 shows a light transmittance of an InGaAsP quaternary mixed-crystal layer having a bandgap wavelength of 1.42 xcexcm. The absorption layer of the PD shown in FIG. 9 is made of such a mixed-crystal. Its transmittance is zero for 1.3-xcexcm light (incoming light). In other words, it absorbs and detects 1.3-xcexcm light completely. On the other hand, its transmittance is almost one hundred percent for 1.55-xcexcm light (outgoing light). In other words, it transmits 1.55-xcexcm light almost completely, which means it does not detect 1.55-xcexcm light. Therefore, a PD as shown in FIG. 9, which has wavelength selectivity, can be used singly as a photodetector at a central office. The PD shown in FIG. 9 has an opening both at the front and back sides, because behind the PD another PD for detecting 1.55-xcexcm light is to be placed. However, when a PD is used at a central office, only one opening is required because the PD has only to absorb 1.3-xcexcm light. If the PD is a back-illuminated type, the front side is covered by the p electrode. If the PD is a front-illuminated type, the back side is covered entirely by the n electrode. Such a PD can be used as a photodetector at a central office without modification.
FIG. 11 shows a back-illuminated type PD conceived on the basis of the above-described consideration for the use in a central office. Although the PD is almost the same as that shown in FIG. 9, it has a slightly different structure in the vicinity of the p electrode. An n-InP buffer layer 61, an n-InGaAsP absorption layer 62 (xcexg=1.42 xcexcm), and an n-InP cap layer 63 are grown epitaxially on an n-InP substrate 60. At the center portion of the chip, Zn is diffused to provide a p-type region 64. A p electrode 65 having no opening is provided to cover almost the entire p-type region 64. Since the front side is not required to admit light, no opening is provided there. At the outside of the p electrode 65, a passivation layer 67 is formed to protect the edge of the pn junction. Since light enters the PD at the back side, the back-side structure is the same as in FIG. 9. A ring-shaped n electrode 68 is formed on the bottom surface of the InP substrate 60. The inside of the n electrode 68 forms an opening for admitting light and is covered by an anti-reflection layer 69. The ring-shaped electrode is provided without overlapping with the opening.
It should be possible to use a PD having such a structure as a photodetector at a central office. Nevertheless, the present inventors found that when such a PD is used as a photodetector at a central office, crosstalk occurs due to the influence of 1.55-xcexcm light (outgoing light at the central office). It was out of the present inventors"" expectations. The InGaAsP absorption layer 62 has a bandgap wavelength of 1.42 xcexcm. Since it is shorter than 1.55 xcexcm, the present inventors expected the absorption layer to be insensitive to 1.55-xcexcm light as an ideal case. However, the result showed differently. The present inventors found that when the absorption layer 62 has a thickness of 5 xcexcm, it detects about 0.2% of 1.55-xcexcm light. The absorption layer absorbs 100% of 1.3-xcexcm light. The fact that the absorption layer detects 1.55-xcexcm light even in small magnitudes poses a problem. Although 1.55-xcexcm light has an energy lower than the bandgap energy, there are some impurity levels in the bandgap, and these levels effect the slight sensitivity to 1.55-xcexcm light. At a central office, there is imbalance in intensity of light. Whereas the 1.55-xcexcm outgoing light generated in the office is intense, the 1.3-xcexcm incoming light having propagated over an optical fiber is weak. The 1.55-xcexcm outgoing light is more intense than the 1.3-xcexcm incoming light by orders of magnitude. Therefore, even the 0.2% sensitivity can produce an un-ignorable magnitude in noise level because the multiplier has a considerable magnitude.
An object of the present invention is to offer a photodiode in which crosstalk caused by the intrusion of intense 1.55-xcexcm outgoing light into the 1.3-xcexcm light detection portion at a central office can be reduced. This reduction in crosstalk can be accomplished by devising the configuration of the photodiode.
The present invention intends to reduce the crosstalk caused by 1.55-xcexcm light by preventing or impeding the return of the outgoing light (xcex2=1.55 xcexcm) to the absorption layer after passing through the absorption layer once. In order to achieve this purpose, a layer for absorbing 1.55-xcexcm light is additionally provided at the inside or at the outside of a PD. In the present invention, this additionally provided absorption layer is called a xe2x80x9cfilter layer.xe2x80x9d Since these filter layers absorb unwanted 1.55-xcexcm light, the light does not return to the absorption layer for 1.3-xcexcm light, or its intensity is notably reduced even if it returns. This measure can effectively reduce the crosstalk to the 1.3-xcexcm light by the 1.55-xcexcm light predominant at a central office. The methods for providing filter layer for absorbing the 1.55-xcexcm light include the following four types:
Type 1: To replace the InP cap layer by a thick InGaAs cap layer;
Type 2: To provide an InGaAs filter layer by epitaxial growth and remove its peripheral region;
Type 3: To laminate an InGaAs filter layer on the p-type region by the selective growing method; and
Type 4: To laminate a filter layer made of a plastic resin or another material on the entire top surface of a chip.
When a material that absorbs 1.55-xcexcm light is provided on the p-type region as mentioned above, the 1.55-xcexcm light (outgoing light) that has once passed through the p-type region does not return to the absorption layer, or it loses its intensity notably even if it returns. The methods are not limited to the above-mentioned four types providing that a newly conceived method can achieve a similar effect.